Shark numbers may not be on the rise: researcher

BEHAVIOUR of great white sharks when a shark cage dive operator is present at Neptune Islands changes dramatically during daylight and causes the sharks to swim closer to the surface, according to researcher Dr Charlie Huveneers.

He said research conducted at the Neptune Islands focused on shark behaviour and they were monitored by an acoustic tag.

He said cage dive operators may be impacting the fine-scale swimming behaviour of the sharks at Neptune Islands.

"More specifically, the size of the area in which white sharks spent 50 per cent of their time was reduced by 28 per cent when shark cage-diving operators were present," he said.

"Sharks also swam at shallower depths during berleying periods."

He also said there were less detections of sharks at night time than during the day.

However the response of sharks to the cage diving boat was not uniform.

"It is important to take into account the actual level of direct interactions between the sharks and the operators."

Australian Southern Bluefin Tuna Industry Association chief executive Brian Jeffriess questioned the increasing number of great white sharks in the area, saying the reduction of gill net catch was a driving factor.

"We have the same problem when the next fatality occurs next to Port Lincoln, and how to explain it," he said.

He asked what was the great white shark population expectation in the future, and when it would reach that size.

Dr Huveneers disputed that there had been a rise in great white shark numbers

"We don't actually know whether the number of white sharks have been increasing and an increase of sightings does not necessarily relate to an increase in population size," he said.

"While a few summers ago, there was a period during which the cage-diving operators did not observe many white sharks, it did not specifically mean that the population size had decreased.

"On the other hand, the large number of sharks sighted at the Neptune Islands in the summer of 2011 did not suggest an increase of population size."

He said the findings may simply mean there were less white sharks in other parts of the ocean.

"Just because we are seeing more sharks in one location doesn't mean the population is increasing," he said.

"Biologically, taking into account their reproductive cycle and age-at-maturity, and considering that white sharks have been protected for about 14 to 16 years, the number of large adults would not yet be expected to have increased significantly.

"If an increase in the white shark population is taking place, it would be through an increase number of neonate and juvenile white sharks."